Reduction of weight of the cross members, side members, and other members of automobiles has been considered so as to deal with the trend toward reduction of weight for improvement of fuel economy in recent years. In terms of materials, from the viewpoint of securing strength and impact safety even if made thinner, steel sheet is being made higher in strength. However, the shapeability of materials deteriorates along with the rise of strength, so to achieve lighter weight of the members, steel sheet which satisfies both shapeability and high strength has to be produced.
As steel sheet which achieves both shapeability and high strength, PLT 1 discloses the art of utilizing residual austenite and using transformation-induced plasticity to improve the ductility, so-called residual austenite steel. However, to enable residual austenite to remain, the cooling rate after dual-phase annealing has to be increased to prevent ferrite transformation and pearlite transformation and Si and Al have to be added to suppress the precipitation of cementite. To make the cooling rate greater, a continuous annealing line with a high cooling rate becomes necessary. Addition of a high content of Si impairs the plateability, while a high content of Al often impairs the castability.
PLT 2 and PLT 3 disclose so-called “dual phase steel” which has a composite structure of low temperature transformed phases which contain ferrite and martensite (hereinafter referred to as “DP steel”). This is being widely used. DP steel exhibits a sufficient strength-ductility balance, through not reaching that of residual austenite steel, so is used for relatively complicatedly shaped chassis parts. Further, the strength of DP steel is increasing along with the trend toward lighter weight of chasses in recent years.
For example, PLT 4 and PLT 5 disclose the art of adding the carbide-forming elements of Nb, Ti, and other elements to suppress recrystallization during annealing and utilize precipitation strengthening so as secure a tensile strength of 780 MPa or more.
Further, PLT 6 and PLT 7 relate to composite structure steels, where the stretch flange formability is generally low in level, and show the art of controlling the difference in hardnesses of the base phase ferrite and the low temperature transformed phases so as to improve the stretch flange formability. In these inventions, the hardness is measured by the Vicker's hardness.
However, in the case of DP steel with a tensile strength of 780 MPa or more, the grain size of the microstructure is small and the hardness cannot be evaluated by the Vicker's hardness. Therefore, art which evaluates properties by the nano hardness which is measured using the newly developed art of nano indentation is disclosed in PLT 8. With this art, the ratio of hardnesses of the ferrite and the low temperature transformed phases is defined in accordance with the ferrite fraction. Due to this, the bending properties are improved.